Soil Erosion
Soil Erosion physical removal of topsoil by various agents, including falling raindrops, water flowing over and through the soil profile, wind velocity and gravitational pull. Geological and accelerated erosion represent contrasting types of soil removal. Geological erosion is a normal process, representing erosion of land in its natural environment without the influence of man. This is a slow and constructive process. The vastly accelerated process of soil removal brought about by human influence with the normal equilibrium between soil building and soil removal is designated as accelerated erosion which has severe adverse effects on soil and environment.
Erosion is manifested by the deterioration of soil surfaces affected by erosive forces, especially water, ice, wind, glacier, etc according to the agent causing the occurrence and affecting the course of the erosion process. Soil erosion may be classified into water erosion, glacier erosion, snow erosion, wind erosion etc. On a global scale, greatest damage to the soil comes from water and wind erosion whose unfavourable effects are increased manifold by anthropogenic agents.
Water erosion is the most widespread form of degradation affecting 25% of agricultural land of Bangladesh. Various kinds of soil erosion such as sheet, rill and gully erosion, landslide, riverbank erosion and coastal erosion are occurring in Bangladesh. Accelerated soil erosion has been encountered in the hilly regions of the country, which occupy about 1.7 million hectares. In a study at the Ramgati station of the bangladesh agricultural research institute (BARI), total soil loss of 2.0 to 4.7 ton/ha per year was observed.
An estimated soil loss is 4.2 tons/ha/yr and 7-120 tons/ha/yr on 30-40% and 40-80% slopes, respectively due to shifting cultivation. Besides soil loss, significant quantities of plant nutrients are also depleted from top layer causing a tremendous soil degradation. In addition, the country is losing its forest area at the rate of about 3% annually due to deforestation. The deforested area is also becoming susceptible to severe water erosion, which is about 102 tons/ha/yr. In Bangladesh, bank erosion is caused due mainly to strong river current during the rainy season.
About 1.7 million hectares of floodplain areas are prone to riverbank erosion. Some areas of Bangladesh are also affected by wind erosion, particularly in the Rajshahi and Dinajpur regions during the dry months of the year. The soils eroded from the hills are deposited somewhere in the downstream. Burial of agricultural land by sandy overwash is a common feature in areas adjoining the active river channels and hill streams. The entire northern and eastern piedmont alluvium and the chittagong hill tracts are adversely affected by the deposition of coarse materials brought down by runoff water.
Soil erosion being irreversible, is generally regarded as the most serious problem of soil degradation. Soil erosion management is based on the following tenets: a) highly erodible or susceptible soils must be protected to prevent accelerated erosion, b) potentially productive soils must be conserved properly to sustain their fertility and c) eroded soils must be rehabilitated while averting their further degradation. [TH Khan, SM Fazle Rabbi and Sultana Razia]
